Renewable energy resources have been gaining prominence due to rising concerns for the environmental impact of more traditional energy sources. Renewable energy is derived from natural resources that may be replenished, unlike fossil fuels. One type of renewable resource is biomass (also referred to as biofuel), which is derived from recently living organisms, as opposed to fossil fuels derived from ancient biological sources. Biomass encompasses various plant and animal matter, including forest (woody) residues; agricultural products, such as, corn stover, straw, and grasses; wastes, e.g., biodegradable wastes or garbage; alcohol fuels; landfill gases; and other organic materials. Because biomass contains carbon, biomass may be combusted to provide energy. Unlike the combustion of fossil fuels, which releases carbon dioxide into the atmosphere that has been stored in the Earth for millions of years, the combustion of biomass releases carbon dioxide that was only recently extracted from the atmosphere, because biomass is derived from recently living organisms. Thus, biomass is essentially carbon dioxide neutral and gentler to the environment than fossil fuels.
Many types of biomass are relatively wet in their raw state for a variety of reasons. For example, various types of biomass derived from plant matter, such as, grasses, naturally absorb water. Additionally, biomass may be wet due to washing prior to combustion to reduce salt content, e.g., to reduce the corrosive effects of the biomass on a combustion chamber or to improve the quality of byproducts of combustion such as ash. Various applications employing biomass as a fuel require the biomass to be processed from a relatively wet state to a dry state, so that the biomass can be used directly in existing combustion apparatuses. A type of thermochemical treatment, known as torrefaction, can be used to convert relatively wet biomass to dry biomass. Torrefaction involves subjecting a material to a high temperature, e.g., in the range from 250-300° C. (482-572° F.) in a low-oxygen or oxygen-free gaseous environment near atmospheric pressure. The absence or near-absence of oxygen prevents the material from burning, while moisture and volatile gases in the biomass are sublimated by the heat. Torrefaction, which can be considered a mild form of pyrolysis (chemical decomposition by heating), results in chemical changes to the material that render the material easier to grind. The solid, torrefied material has a higher relative energy content than the original biomass. Additionally, because the torrefied material is hydrophobic, it is relatively resistant to rotting.
Improved techniques for torrefaction, including techniques making more efficient use of available energy resources, are desired.